Current wireless communication systems are typically either Frequency-Division Duplex (FDD) systems or Time-Division Duplex (TDD) systems. In FDD operation there are two carrier frequencies, one for uplink transmissions (user terminal to base station) and one for downlink transmissions (base station to user terminal). Hence, in FDD systems, uplink and downlink transmissions occur simultaneously but in different frequency bands. In TDD operation there is a single carrier frequency only and the uplink and downlink transmissions are separated in the time domain (e.g. occur in different system time slots). Hence, in TDD systems, uplink and downlink transmissions occur in the same frequency band but not simultaneously.
The respective standards that define cellular wideband code division multiple access (WCDMA) systems (so called 3G systems) and cellular long term evolution (LTE) systems (so called 4G systems) specify system variants based on both FDD and TDD. Wireless local area networks (WLAN), for example WiMAX, tend to be TDD systems only.
Herein, communication systems in which it is not possible to transmit and receive simultaneously in the same frequency band will be referred to as half-duplex or non full-duplex systems. In contrast, communication systems in which it is possible to transmit and receive simultaneously in the same frequency band (e.g. with no frequency duplex spacing between the transmission and reception bands) will be referred to as full-duplex systems. A frequency band refers to a frequency allocation assigned to a baseband signal after being up-converted on a certain carrier frequency.
Full-duplex communication has been recognized as a candidate technique for improving spectral efficiency in local area communications. In full-duplex communication systems techniques must be applied to reduce strong ‘self-interference’ that would otherwise occur as a result of the large power imbalance between the simultaneously transmitted signal and received signal. Typically, the transmitted signal can have a signal strength that is a few orders of magnitude larger than that of the intended received signal strength with the result that the intended received signal can be severely degraded by the transmitted signal.
Known techniques for reducing self interference at a full duplex transceiver can be broadly categorised as techniques that rely on the linear processing of antenna arrays at the transceiver to create a transmission null at the transceiver's receiver (referred to herein as linear space domain interference processing techniques) as discussed for example in references [1] or techniques that don't rely on the linear processing of antenna arrays at the transceiver but various other techniques (referred to herein as non-linear interference processing) such as those disclosed in references [2] to [6] listed below:    [1] Riihonen, T.; Werner, S.; Wichman, R.; “Mitigation of Loopback Self-Interference in Full-Duplex MIMO Relays,” Signal Processing, IEEE Transactions on wireless communications, vol. 59, no. 12, pp. 5983-5993. December 2011.    [2] Jung II Choiy, Mayank Jainy, Kannan Srinivasany, Philip Levis, Sachin Katti, “Achieving Single Channel, Full Duplex Wireless Communication”, In the Proceedings of the 16th Annual International Conference on Mobile Computing and Networking (Mobicom 2010).    [3] Melissa Duarte and Ashutosh Sabharwal, “Full-Duplex Wireless Communications Using Off-The-Shelf Radios; Feasibility and First Results”, in the proceedings of the 44th annual Asilomar conference on signals, systems, and computers 2010.    [4] Melissa Duarte, Chris Dick and Ashutosh Sabharwal, “Experiment-driven Characterization of Full-Duplex Wireless Systems”, Submitted to IEEE Transactions on Wireless Communications, July 2011.    [5] Evan Everett, Melissa Duarte, Chris Dick, and Ashutosh Sabharwal, “Empowering Full-Duplex Wireless Communication by Exploiting Directional Diversity”, accepted to the 45th annual Asilomar conference on signals, systems, and computers 2010.    [6] Achaleshwar Sahai, Gaurav Patel and Ashutosh Sabharwal, “Pushing the limits of Full-duplex: Design and Real-time Implementation”, Rice university technical report TREE1104.
LTE and WiMAX communication systems, amongst others, employ Multiple Input Multiple Output (MIMO) techniques to enhance data rates (throughput) and spectral efficiency. As is well known, MIMO refers to the use of multiple antennas at the receiver and multiple antennas at the transmitter. Different modes of MIMO are known, including spatial multiplexing, transmit diversity, and beam forming.
Spatial multiplexing allows the transmission of different independent layers or streams of data simultaneously on the same downlink (or, as the case may be, uplink) resources to increase data rates for a given channel bandwidth. Transmit diversity schemes increase the resilience of a propagation channel, rather than increasing data rates, and are often used when channel conditions do not permit spatial multiplexing. Beam forming techniques provide for the shaping of the overall antenna beam in the direction of the target receiver and again are often used when channel conditions do not permit spatial multiplexing.
In a MIMO system, each receive antenna may receive the signal transmitted from each transmit antenna and the channel between the transmitter and the receiver can be described by a channel matrix H including the matrix elements hij, where hij is the channel gain from transmit antenna j to receive antenna i. If spatial multiplexing is employed, the number of independent data layers or streams that can usefully be transmitted in parallel over a MIMO channel is at most the minimum of the number of receive antennas and the number of transmit antennas, and is further limited by the rank (i.e. the minimum number of linearly independent rows and columns) of the channel matrix H. In certain known communications systems, for example, LTE systems, spatial multiplexing relies upon a user device (i.e. a mobile user equipment) providing periodic feedback to the base station (i.e. Node B) serving it. The feedback includes a so called Rank Indication (RI) determined by the user device, based on channel and interference conditions, which indicates a suggested number of layers for transmission on the downlink to the user device. The base station may or may not follow the suggestion provided by the user device.
In environments where the transmission antennas are not sufficiently de-correlated, for example in a low scattering environment (i.e. where there are few or no objects to reflect signals from the different antennas to the different receive antennas along different paths), the rank of the channel matrix H is low and the potential throughput advantages of spatial multiplexing cannot be utilized effectively. Typically, an environment is not sufficiently scattering enough to warrant the use of spatial multiplexing if a Line of Sight (LOS) exists between the transmitter and the receiver in a MIMO system which can result in the rank of the channel matrix reducing to 1. In known systems it is possible to switch between MIMO modes in response to changing channel conditions, and typically, for rank 1 channels, transmit diversity or beam forming is used instead of spatial multiplexing. It is well-known that for rank one channels the maximum throughput of a half duplex system is limited by the performance of the rank one transmit diversity or beam forming pre-coding performed at the transmitter, power allocation to the channel and the modulation and coding scheme (MCS) selected for the channel.
Wireless systems have been proposed in which it is possible to selectively use a full duplex operation mode instead of a half duplex mode, under certain channel conditions, in order to improve throughput. It is desirable to provide a system in which the signalling schemes used to configure devices in a full duplex operation mode or a half duplex operating mode and/or schedule the devices are straightforward.